What factors are used to determine the visibility for a category B airplane during an ILS approach?

For a category B airplane during an ILS (Instrument Landing System) approach, the visibility is primarily determined by the published ILS visibility minimums. These minimums are established based on several operational factors and safety considerations to ensure that aircraft can safely execute the approach and land.

Visibility minimums are specified in the approach plate for each ILS procedure and take into account the specific characteristics of the airplane, such as its approach speed and operational capabilities. Since category B airplanes have defined speed categories, the published minimums reflect the level of visibility needed for safe operation which ensures pilots can adequately see the runway environment at the decision height.

In contrast, while weather conditions during the approach can affect actual visibility, they do not establish the minimums; those are predefined and must be adhered to. The runway length is not a determining factor for visibility rules during the ILS approach, as it pertains more to other safety and operational considerations. Control tower directives may influence the approach process but do not pertain to the formal requirements for visibility that are laid out in the ILS approach charts.